Food safety within the food industry is of utmost importance as food contamination or deterioration impacts both health (food poisoning) and overall quality (taste and texture) of the food product. Current technologies aim to provide long-lasting freshness of a food product. For example, the primal cuts of chilled, vacuum-packaged beef can have a shelf life of 30 weeks and beyond.
In some countries, there are legal requirements for every basic packaging unit of meat or imported meat products to be labelled. The labels should state, among other details, the date on which the meat product was packed, the shelf life (or its best before date or the expiration date). The indicated dates are usually supported by experimental data but are always averaged. Thus, even if the quality of that food product remains acceptable and safe for consumption after the expiration date, its sale would not be allowed.
On the other hand, whether each product will achieve its shelf life is dependent upon initial quality of the meat (pH, microbiological quality), the integrity of the packaging, the presence of adequate temperature control (minus 0.5±0.5° C.), etc. For meat products that are left unpacked for an extended period after slaughter, even vacuum-packaging may not provide an extended shelf life because of the decline in activity of certain enzymes in the meat which assist the consumption of oxygen present in the packaging head space.
There are generally two types of methods used to evaluate food freshness/spoilage. One is a rapid but expensive and subjective test of the organoleptic attributes. This method becomes less reliable when applied to the inspection of processed food. The other is the detection of certain biomolecules/bio-markers of food spoilage due to autolysis or bacterial growth using chemical assays. This method is time-consuming, destructive and requires sophisticated equipment, highly skilled operators, etc. Both these methods are applied for inspection of some chosen samples from a whole consignment.
These methods are generally suitable for food safety agencies, but may not be suitable for use in supermarkets, consumer homes and may not be applicable towards every form of packaged meat/poultry/seafood (“food products”).
There is thus a need for a rapid and non-destructive method of evaluating freshness/spoilage of food products. Preferably, it is desired that the method is capable of inspecting a large number of samples on-site by scanning through the packaged product. Such methods are expected to improve consumer confidence in the food products even if the indicated shelf-life may be close to expiring.
A number of methods/devices have been developed to evaluate freshness and detect spoilage of food products. In general, there are four groups of such methods. The first group is time-temperature indicators. Temperature is an important environmental factor influencing the kinetics of physical and chemical deteriorations, as well as affecting the rate of microbial growth in food products which is pertinent to chilled or frozen food products. Time-temperature indicators report temperature history during transportation and storage. These indicators may change color in response to the intensity of the temperatures the food product was exposed to and the duration of exposure.
Radio-frequency identification technology (“RFID tagging”) helps to identify a product, its manufacturing date, country of origin, etc. Temperature monitoring RFID tags have been developed for assessing freshness of food products. These RFID tags have a microchip for sensing temperature changes over time, and recording data throughout the supply chain journey of the food product. At various key points, a prediction of the food product's remaining shelf-life can be made based on the recorded data. Initial product quality may be recorded for each consignment of food product to establish the parameters for the shelf-life prediction.
The second group is gas indicators signalling the gas composition in the package headspace. Of particular prominence are oxygen indicators as it is typically O2 that causes oxidative rancidity, color-changes and also leads to microbial spoilage of foods. Other useful gas indicators may be capable of detecting and measuring water vapor, carbon dioxide, ethanol, hydrogen sulfide, and other gases.
The third group is biosensors that detect and identify pathogens and monitor post-processing food quality parameters. Usually, these biosensors contain a specific-pathogen antibody that can recognize and report the presence of a specific target analyte, e.g., contaminating bacteria.
The fourth group is indicators that report chemical changes within a food product during microbial growth. An increase in pH in meat/poultry/seafood juice upon storage may be the result of the formation and accumulation of biogenic amines due to enzymatic amino acids decarboxylation. Some bacteria which are inhibited at pH 5.4-5.7 can grow to spoilage levels at a higher pH, thus consuming amino acids and producing even more biogenic amines hence further increasing pH in a positive feedback loop. Such biochemical changes occur when spoilage has commenced, and can be indicative of a medium- or a late-stage of the food spoilage process.
K-index measurement is based on measuring ATP breakdown products formed from the initial biochemical processes occurring once an animal or fish has died, but long before spoilage begins. Measurement of the K-index allows anticipation of the beginning of spoilage and to better control the freshness level of the product. K-index can be used as a freshness marker at an early stage of the food spoilage process. Moreover, the K-index also determines the quality of food produce as it also correlates to the major component of the savoury (“umami”) taste.
Other detectable metabolites are for instance organic acids such as n-butyrate, L-lactic acid, D-lactate and acetic acid; ethanol; biogenic amines such as histamine, putrescine, tyramine and cadaverine.
However, at this time, known time-temperature indicators, gas sensors or analyte-sensors are unable to provide comprehensive information about the complex biochemical processes occurring in food upon ageing depending on the storage time, temperature, packaging conditions, microbial loading, humidity, etc.
The development of multi-sensors for the quantitative, rapid and concurrent detection of different analytes is one of the major challenges in analytical chemistry.
A majority of existing multi-sensors called “electronic noses” were developed for analysis of gases. More recently, multi-sensors called “electronic tongues” have been developed for liquid analysis through the use of a multivariate interpretation of signals coming from a set of electrodes. Electronic tongues have been applied for food freshness analysis by measuring both pH and K-index. However, these methods require analysing samples obtained from crushed meat or sticking a set of electrodes directly onto a meat product to be tested, i.e., these are destructive methods and assessment cannot be made remotely.
Optical sensing systems can be useful for food freshness/spoilage assessment as they enable rapid and non-destructive scanning of samples on-site remotely, through the common packaging material. Among optical methods, fluorimetry is a promising analytical tool as it provides high sensitivity and ability for simultaneous detection of multiple-analytes at low cost.
Hence, it has been contemplated to provide optical sensing elements in food packaging material.
For instance, it has been suggested to encapsulate fluorescent dyes in a solid polymer matrix. In particular, the fabrication of such sensors generally involves dissolution of lipophilic sensing dye and an appropriate polymer support in an organic solvent. This solution is then applied to a solid substrate like polyester film or glass and allowed to dry. A number of coating techniques like casting, spin coating, dipping have been used to produce a thin film of dye polymer coating.
In another approach, sensing dyes are absorbed in mesoporous inorganic particles, e.g. silica or alumina. A dye solution in the appropriate solvent, e.g., dichloromethane or ethanol, is added to a suspension of mesoporous particles for 24 h followed by solvent removal. The powder containing 2-8 wt. % of a dye is placed into a microplate which is packed together with meat in polystyrene boxes. In both methods, vapours of an analyte (oxygen or volatile compounds generated during meat spoilage) penetrate the solid matrix through simple diffusion. That is, these sensors work as “optical noses”, which analyse and detect the presence of gases and other volatile compounds in the headspace of food packaging.
The drawback of such sensors is that they cannot be applied to packaged foods with insufficient or no headspace, e.g., foods that are vacuum-packed. Also, gas detection (e.g., CO2) is not a comprehensive method of assessing freshness. Such sensors also provide little information as to the extent of food spoilage or the taste quality (e.g., the umami taste) of the food product. Furthermore, by placing these sensors in close proximity to the food product, there is also a risk of contaminating the food products with the chemicals present in these sensors.
Oxygen-sensitive fluorescent dyes have also been indicated for use in sensing applications. In one known example, oxygen-sensitive fluorescent dyes are encapsulated within a polymer matrix that is substantially permeable to oxygen. The polymer matrix is then applied onto food packaging material. The encapsulation process is however not straightforward and there is little control over the amount of dyes that are contained within the polymer matrix. It is further considerably challenging to control the permeability of the polymer matrix.
Accordingly, the methods and sensors discussed above are not adequate for providing a wholesome assessment of food quality or freshness. These techniques are mainly interested in the detection of a particular volatile compound (e.g., amine), or a particular gas (O2 or CO2), or the change in a particular property (e.g. pH). However, these indicators, when individually assessed and measured, may be insufficient for ascertaining the freshness of the food product. For instance, some food may not display an appreciable change in pH despite having undergone spoilage. In such cases, a sensor that is capable of measuring the K-index would be able to detect spoilage way in advance of the pH sensor.
Other techniques for testing food products include, e.g., the use of potentiometric probes or food sample testing via chromatographic techniques e.g., high pressure liquid chromatography (“HPLC”). However, all these techniques suffer from a need to obtain a food samples or otherwise having to destroy the packaging of the food product before testing can be conducted. Such techniques do not permit a consumer prior to purchase or for a retailer to assess the quality/freshness/spoilage of the product without making the product unsellable.
Accordingly, there is a need to provide a sensor that overcomes or at least ameliorates the disadvantages or drawbacks discussed above. There is further a need to provide a method for making such sensors.